MICROBIOLOGY OF ALCOHOL AND ALCOHOLIC PRODUCTS. 431 



Some grapes, owing to their composition, especially their high 

 acidity, are very resistant to the attacks of injurious bacteria. Others, 

 owing to their low acidity or highly nitrogenous nature, are very suscepti- 

 ble. The addition of tartaric or citric acid to the latter has therefore a 

 deterring effect on some of the most dangerous forms. It is seldom 

 necessary, however, to modify the composition for this purpose if the 

 other means of control are used. The addition of acid or its decrease 

 by dilution should be solely for the direct improvement of the taste. 



The quality and character of the wine depends greatly on the tem- 

 perature of fermentation. If too low, the fermentation may be unduly 

 prolonged, the wine yeast may have difficulty in overcoming its com- 

 petitors and the wine may remain inferior and cloudy. With red wine, 

 the desired color, tannin and body may not be secured. On the other 

 hand, if the temperature is too high the results are worse. The growth 

 of bacteria is promoted, injuring the wine by the volatile acid and dis- 

 pleasing flavors produced and preventing the proper action of the yeast. 

 Such wines may remain sweet on account of the failure of the yeast to 

 do its work and become unpleasantly acid owing to the volatile acids 

 produced by the bacteria. 



Some means of controlling the temperature is therefore always needed. 

 Where heat is deficient it may be supplied by direct heating of the must 

 or part of it, or by heating the cellar. Where the heat is excessive, it 

 may be diminished by crushing only cold grapes, using small fermenting 

 vats to promote radiation and finally by the use of cooling machines 

 applied directly to the fermenting wine. 



The best temperature for fermentation depends on the kind of wine. 

 For light white wines, the maximum should not exceed 25, for heavier 

 wines 30, while for heavy red wines where high extract and tannin are 

 required, it may be allowed to reach 35. Sound wines can be made at 

 all these temperatures. 



As already explained, the ordinary processes of treatment of grapes 

 result in sufficient aeration for the multiplication of the yeast. With 

 grapes containing little sugar, this may suffice to complete fermentation. 

 With sweeter grapes, the fermentation usually slackens when the alcohol 

 reaches n or 12 per cent by volume or sooner, unless some supplementary 

 aeration is given. With white wine this is seldom done, with the result 

 that the time of fermentation is prolonged. With red wine, the necessary 

 stirring of the pomace to promote color extraction or the pumping over of 



